bugfix: fix the behavior of MLA kernel when kv-length is 0 (#868)

The scheduling algorithm in #863 do not consider some requests have
kv-cache length 0, this PR fixes the issue.

include/flashinfer/attention/scheduler.cuh

@@ -1134,7 +1134,8 @@ inline cudaError_t MLAPlan(void* float_buffer, size_t float_workspace_size_in_by
             qo_indptr_h[i] * num_heads +
             std::min((qo_tile_idx + 1) * cluster_tile_q, packed_qo_len);
       }
-      while (remaining_len > 0) {
+      bool zero_kv_len = (remaining_len == 0);
+      while (remaining_len > 0 || zero_kv_len) {
         auto [cluster_idx, accum_cost] = cluster_cost_heap.pop();
         int actual_len = std::min(remaining_len, kv_len_limit);
         cluster_cost_heap.insert(
@@ -1154,6 +1155,7 @@ inline cudaError_t MLAPlan(void* float_buffer, size_t float_workspace_size_in_by
         cluster_kv_end[cluster_idx].push_back(kv_start + actual_len);
         remaining_len -= actual_len;
         kv_start += actual_len;
+        if (zero_kv_len) break;
       }
       split_kv_count += int(split_kv);
     }

tests/test_deepseek_mla.py

@@ -171,7 +171,7 @@ def generate_kv_from_cache(ckv, kpe, kv_len, batch_size, num_heads):
 
 
 @pytest.mark.parametrize("batch_size", [1, 17, 37])
-@pytest.mark.parametrize("kv_len", [17, 33, 96, 97, 114, 514, 1024])
+@pytest.mark.parametrize("kv_len", [0, 17, 33, 96, 97, 114, 514, 1024])
 @pytest.mark.parametrize("qo_len", [1, 17, 37, 77])
 @pytest.mark.parametrize("num_heads", [4, 32, 128])
 @pytest.mark.parametrize("causal", [False, True])
@@ -243,7 +243,8 @@ def test_batch_mla_page_attention(
     o_ref, lse_ref = attention_ref(batch_size, q, k, v, causal, sm_scale)
     lse_ref = lse_ref.flatten(0, 1)
     torch.testing.assert_close(o, o_ref, rtol=1e-3, atol=1e-3)
-    torch.testing.assert_close(lse, lse_ref, rtol=1e-3, atol=1e-3)
+    if kv_len != 0:
+        torch.testing.assert_close(lse, lse_ref, rtol=1e-3, atol=1e-3)
 
     # test with pre-allocated output
     o_buffer = torch.empty_like(o)